Fix arm64 superpages bugs in pmap_enter:

• Pass the correct virtual address when demoting a superpage
• Use the correct l3 table after demoting a superpage
• Remove an invalid KASSERT hit demoting then promoting a superpage [1]

With this it is believed that superpages on arm64 is stable.

Reported by: [1] cognet
Obtained from: ABT Systems Ltd
MFC after: 1 week
Sponsored by: The FreeBSD Foundation